The present invention relates to optical imaging systems, and more particularly, to correcting reverted optical images.
Optical imaging systems that include an odd number of mirror or prism reflections produce reverted images. A reverted image is rotated 180 degrees about a first axis, such as a horizontal axis, but is not rotated about a second axis that is orthogonal to the first axis, such as a vertical axis. The familiar xe2x80x9cmirror imagexe2x80x9d one observes when looking at a mirror is an example of a reverted image. A xe2x80x9cmirror imagexe2x80x9d is rotated about a vertical axis but is not rotated about a horizontal axis that is coplanar with and orthogonal to the vertical axis. In the same manner, the reflection off the surface of a body of water is rotated about a horizontal axis but is not rotated about a vertical axis. Such a xe2x80x9cpond reflectionxe2x80x9d is also a reverted image. In some applications, such as in avionics displays, it may be necessary to correct such an image reversion.
In contrast to a reverted image, an inverted image is one that is reverted in both axes when viewed. For example, an astronomical telescope usually presents an inverted image that is reversed in both horizontal and vertical axes. An inverted image appears to be rotated 180 degrees about the optical axis, which is perpendicular to both the horizontal and vertical axes of the image. Inverted images are corrected by known xe2x80x9cerectingxe2x80x9d prisms or lenses.
Reverted images can be corrected by introducing a second reversion that cancels the original reversion. Known reversion correction systems use prisms or systems of planar mirrors to create the second reversion. However, prisms are difficult to fabricate, heavy, and introduce image aberrations when used with converging or diverging beams. On the other hand, mirror systems are difficult to maintain in alignment, especially in high-vibration environments. Some prisms and mirror systems also introduce lateral offsets in beam position, changes in beam direction, and/or rotation about the optical axis. These effects diminish image quality and render the systems unsatisfactory for some uses absent additional corrective components.
It is therefore an object of the invention to provide a way to correct a reverted image without using prisms or mirrors.
It is another object of the invention to correct a reverted image using easily manufactured components.
It is another object of the invention to correct a reverted image in a manner that is reliable in high-vibration environments.
It is still another object of the invention to correct a reverted image such that a high-quality corrected image is provided.
It is yet another object of the invention to correct a reverted image using a minimum of space.
A feature of the invention is the use of a plurality of coaxially aligned lenses to focally or afocally correct a reverted image.
An advantage of the invention is that a smaller, more reliable reversion corrector is provided that produces a high-quality corrected image without additional corrective components.
The invention provides an apparatus for correcting a reverted image. The apparatus includes first, second, and third axially aligned lenses. Each of the first and third lenses have a first focal length in a first plane and a second focal length in a second plane that is orthogonal to the first plane. The second lens is disposed between and coaxially aligned with the first and third lenses. The second lens has a third focal length in the first plane and a fourth focal length in the second plane. The reverted image passes through the first, second, and third lenses and is reversed along one of the first and second planes to thereby correct the reverted image.
In another aspect of the invention, an apparatus is provided that afocally corrects a reverted image that is projected from a first location along an optical path to a second location. The apparatus includes first, second and third lenses coaxially aligned along the optical path. Each of the first and third lenses are convex in a first plane and in a second plane that is orthogonal to the first plane. The second lens is disposed equidistant from and coaxially aligned with the first and third lenses along the optical path. The second lens is concave in the first plane and is planar in the second plane. The reverted image passes through the first, second, and third lenses and is reversed along the second plane to thereby afocally correct the reverted image.
The invention also provides a method of correcting a reverted image in a display system. According to the method, a first convex lens and a second convex lens are axially aligned along an optical path. A third lens is axially aligned between the first and second lens and along the optical path. The third lens has a finite focal length in a first plane and an infinite focal length in a second plane that is orthogonal to the first plane. A reverted image is projected along the optical path. The reverted image is reversed by at least one of the first, second and third lenses in one of the first and second planes to thereby correct the reverted image.